fchownat.2: Document AT_EMPTY_PATH

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/fchownat.2

@@ -88,8 +88,25 @@ is absolute, then
 .I dirfd
 is ignored.
 
+The
 .I flags
-can either be 0, or include the following flag:
+argument is a bit mask created by ORing together
+0 or more of the following values;
+.TP
+.BR AT_EMPTY_PATH " (since Linux 2.6.39)"
+.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d
+If
+.I pathname
+is an empty string, operate on the file referred to by
+.IR dirfd
+(which may have been obtained using the
+.BR open (2)
+.B O_PATH
+flag).
+In this case,
+.I dirfd
+can refer to any type of file, not just a directory.
+
 .TP
 .B AT_SYMLINK_NOFOLLOW
 If